Method and Apparatus for Supervisory Circuit for Ground Fault Circuit Interrupt Device

ABSTRACT

An apparatus and method for a supervisory circuit for a ground fault detection device or a ground fault circuit interrupt (GFCI) device is disclosed in which a low voltage DC power supply is used to generate a test stimulus signal for a self test of the GFCI device. The GFCI device includes line and neutral conductors configured to connect an AC power source and a load. A differential current transformer includes a toroid, through which the line and neutral conductors pass, and a secondary winding wound on the toroid. A differential ground fault detector is electrically connected to the secondary winding of the differential current transformer to compare current generated in the secondary winding from an imbalance of magnetic flux in the toroid to a trip threshold. A wire conductor is routed through the toroid of the differential current transformer. A controller is configured to control a low voltage DC test stimulus signal to be generated in the wire conductor.

This application claims the benefit of U.S. Provisional Application No.61/311,955, filed Mar. 9, 2010, the disclosure of which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to ground fault detection circuits andmore particularly, to testing ground fault detection circuits using astimulus signal generated using a low voltage DC power supply.

Ground Fault Circuit Interrupt (GFCI) devices detect the presence ofground current faults and grounded neutral faults, and interrupt powerin AC power systems if such faults are detected. Accordingly, GFCIdevices provide protection from electrocution and are typically used inreceptacles in kitchens, bathrooms, and outdoor receptacles where theremay be water or moisture that can pose a risk of electrocution. GFCIdevices are also used in circuit breakers that protect these same areasof residential buildings. GFCI devices and other devices that detectground faults and interrupt AC power systems may also be referred togenerally as “ground fault detectors”. Ground fault detectors typicallyhave supervisory circuits or test circuits that check the functionalityof the ground fault detection circuit.

Ground fault detectors disconnect a circuit when current leakage isdetected. Current leakage occurs when current flowing through a line, or“hot” conductor, from a source load is diverted to ground withoutreturning to the source. This leakage may result from an accidentalshort circuit, such as from a defective load attached to the line. If aperson touches the load, the leakage current may pass through theperson's body to ground, leading to an electric shock. Consequently,ground fault detectors, or GFCIs, act as safety devices and are designedto detect line-to-ground shorts and disconnect the distribution circuit.

Ground fault detectors also need to act quickly. While a typical circuitbreaker interrupts a circuit at 20 amperes, it only takes approximately100 milliamperes to electrocute a person. Therefore, for added safety,ground fault detectors must be able to detect current flow between aline and ground at current levels as little as 6 milliamperes and trip abreaker at the receptacle or at the breaker panel to remove the shockhazard. Ground fault detectors are typically required for receptacles inbathrooms and other areas exposed to water in order to prevent deadlyground fault situations from occurring.

In two-line systems, GFCIs typically detect current leakage by comparingthe current flowing in the line and returning in the neutral. Adifference in current levels implies that some current has leaked fromthe circuit and a ground fault exists. GFCIs typically use adifferential transformer to detect a difference in the current levels inthe line and the neutral. The differential transformer is often atoroidal core that has as its primary windings the line and neutralconductors of the distribution circuit being protected, which areencircled by the core. The secondary windings of the transformer arewrapped around the core. During normal conditions, the current flowingin one direction through the line conductor will return in the oppositedirection through the neutral conductor. This balance produces a netcurrent flow of zero through the differential transformer, and themulti-turn winding provides no output. If a fault exists, current leaksfrom the line conductor to ground and the current flowing back throughthe line and neutral conductors in the differential transformer will notbe equal. This current imbalance will produce uncanceled flux in thedifferential transformer's core, resulting in an output from themulti-turn secondary winding. Detection circuitry identifies the outputfrom the differential transformer and opens the circuit breakercontacts.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for a supervisorycircuit in a ground fault circuit interrupt (GFCI) device. Embodimentsof the present invention utilize a low voltage DC power supply togenerate a stimulus signal, which is used for a self test of a GFCIdevice.

In one embodiment of the present invention, a GFCI device includes lineand neutral conductors configured to connect an AC power source and aload. A differential current transformer includes a toroid, throughwhich the line and neutral conductors pass, and a secondary windingwound on the toroid. A differential ground fault detector iselectrically connected to the secondary winding of the differentialcurrent transformer to compare current generated in the secondarywinding from an imbalance of magnetic flux in the toroid to a tripthreshold. A wire conductor is routed through the toroid of thedifferential current transformer. A controller is configured to controla low voltage DC test stimulus signal to be generated in the wireconductor.

According to another embodiment of the present invention, in a method ofperforming a self test by a GCFI device, a test stimulus signal isgenerated in a wire conductor routed through a toroid of a differentialcurrent transformer from a low voltage DC power supply. It is thendetermined whether a differential current greater than a trip thresholdis detected in the toroid.

These and other advantages of the invention will be apparent to those ofordinary skill in the art by reference to the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional GFCI device with a supervisory testcircuit;

FIG. 2 illustrates a signal timing diagram of the push-to-test self testfor the GFCI device of FIG. 1;

FIG. 3 illustrates a signal timing diagram of an automatic self test forthe GFCI device of FIG. 1;

FIG. 4 illustrates a GFCI device according to an embodiment of thepresent invention;

FIG. 5 illustrates a signal timing diagram of the push-to-test self testfor the GFCI device of the embodiment of FIG. 4;

FIG. 6 illustrates a signal timing diagram of an automatic self test forthe GFCI device according to the embodiment of FIG. 4;

FIG. 7 illustrates a GFCI device according to another embodiment of thepresent invention;

FIG. 8 illustrates a signal timing diagram of the push-to-test self testfor the GFCI device of the embodiment of FIG. 7;

FIG. 9 illustrates a signal timing diagram of an automatic self test forthe GFCI device according to the embodiment of FIG. 7;

FIG. 10 illustrates a GFCI device according to another embodiment of thepresent invention;

FIG. 11 illustrates a GFCI device according to another embodiment of thepresent invention;

FIG. 12 illustrates a GFCI device according to another embodiment of thepresent invention;

FIG. 13 illustrates a GFCI device according to another embodiment of thepresent invention; and

FIG. 14 illustrates a method of performing a self test by a GFCI deviceaccording to an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention relates to a supervisory circuit for testingGround Fault Circuit Interrupt (GCFI) devices. Many GFCI devices have a“test” button for verifying the health of a device. Test methods maycreate a small imbalance by passing a stimulus signal current throughthe core of the differential transformer. For example, pressing the testbutton may cause the 120 volt AC power supply to be drawn across a 15 Kresistor along a test wire that passes through the differentialtransformer. In this example, a current of 8 mA (milliamperes rms),which is greater than the 6 mA leakage current detection requirement forGFCI circuits, passes through the differential transformer. Thedifferential transformer and detection circuitry in a properlyfunctioning device would detect the test current as an imbalance andcause the circuit to trip. The tester interprets this result as meaningthe circuit breaker device is working safely and correctly. If thecircuit breaker does not trip, the tester may assume the circuit has aproblem that may be dangerous and require a specialist's attention andpossible replacement of the device. Some GFCI devices also include areset button for resetting the breaker after it has tripped.

In the near future, many GFCI devices may also include auto self testfunctions that are initiated internally at periodic intervals to verifythe health of the device. Such devices consist of a timer and anelectromechanical or an electronic switch that electrically connects the120 volt AC supply across a resistor along a test wire that passesthrough the differential transformer.

The present inventors have recognized the following problems withconventional supervisory test circuits, whether the test circuit isclosed using mechanical switch or an electronic switch. First,conventional test circuits are directly exposed to noise that exists onthe line conductor of the AC supply, which can possibly interfere withthe test. Second, the AC supply can fluctuate in voltage resulting in arather large variance of stimulus signal current creating test escapeconditions. Third, the test circuit dissipates about 1 watt of powerwhen exercised from a 120 volt AC supply.

With automatic self tests and the use of an electronic switch, the powerdissipation of the test circuit becomes more of a problem. Anotherproblem using electronic switches is direct exposure to high voltages onthe line conductor of the AC supply. Components with higher rated powerand higher rated voltage are larger in size and cost more thancomponents with lower rated power and lower rated voltages.

Embodiments of the present invention provide a supervisory circuit totest ground fault detection circuits by implementing a low voltage DCpower supply from which to directly generate a test stimulus signal,typically a current, instead of directly from the line conductor of the120 volt supply. A low voltage DC power supply provides filtering of anynoise present on the line conductor of the 120 volt supply, thuspreventing interference with the test from the test stimulus signalitself.

A low voltage DC power supply is typically regulated to withinapproximately 100 millivolts to provide a constant DC output voltage,thus eliminating any problems caused by large fluctuations in voltage ofthe AC supply. As a result, the test stimulus signal's current amplitudedoes not need to be set 15 to 20 percent higher than the rated tripthreshold of the ground fault detection circuit to ensure sufficientamplitude, such as when the 120 volt AC supply is 85 percent below ratedvoltage. This is a test requirement in Underwriters Laboratories (UL)943 that governs GFCI devices in residential homes. Using a low voltageDC power supply, the test stimulus signal amplitude can be set to closerto the 6 mA rated trip threshold, thus resulting in more consistentrepeatable tests that eliminate any possible test escape of a degradingtrip threshold.

Power dissipation is reduced in embodiments of the present invention byusing a highly efficient switching DC power supply. Power dissipated inthe test circuit using a 120 volt AC power supply is approximately 1watt (120 volts×8 mA=0.96 watts) every time the test circuit isexercised. If an 80 percent efficient switching DC power supply isimplemented supplying 5 volts DC and 8.5 milliamperes of DC current(peak amplitude of 6 mA rms sinusoid), then the power dissipated in thetest circuit is approximately 53 milliwatts (5 volts×8.5 mA/0.80) everytime the test circuit is exercised. Accordingly, in various embodimentsof the present invention, components with lower rated power may beutilized in the supervisory test circuit with increased reliability dueto lower electrical stress. This may not be much of a concern fordevices with push to test only since the test circuit may only beexercised approximately 240 times over the life of the device if thetest circuit is exercised once per month as recommended. However, fordevices with auto self test, the test circuit may be exercised over50,000 times over the life of the device.

Electronic switches in test circuits that generate the test signalstimulus directly from the line conductor of a 120 volt AC supply aresubjected to transient voltages as high as 400 to 500 volts on the lineconductor of a 120 volt AC supply, even with properly designed transientprotection circuits that include components such as a transorb ormetal-oxide varistor. These transient voltages can be reduced to just afew volts using a DC power supply to generate the test stimulus signal.Hence, an electronic switch and any other components with a much lowervoltage rating may be utilized in the supervisory test circuit withincreased reliability.

Many ground fault detection devices already utilize a low voltage DCpower supply to provide power for the ground fault detection electroniccircuits. According to various embodiments of the present invention, itmay be convenient to use the same low voltage DC power supply todirectly provide the stimulus signal for the test circuit instead of theline conductor of the 120 volt AC supply. Accordingly, since an additionof a low voltage DC power supply is not required for many devices,embodiments of the present invention can be implemented with little orno extra cost or size to the device. Various embodiments of the presentinvention would likely reduce the size and cost since embodiments of thepresent invention utilize components with lower rated power and lowerrated voltages than components used in conventional devices. Componentswith lower rated power and lower rated voltage are generally smaller insize and cost less than components with higher rated power and higherrated voltage. Further, an electronic switch with lower rated voltage,and a resistor that sets the current amplitude of the test stimulussignal with a lower power rating could be integrated into a CMOSdetection ASIC, saving both size and cost. This can be implementedwithout adding an extra pin to the ASIC by reusing the pin for thecontrol signal to the electronic switch.

Ground fault detection devices with supervisory test circuits are notjust found in GFCI outlets or GFCI circuit breakers in residentialbuildings. These devices also have applications in protecting commercialand industrial electrical circuits. These devices may be combined withother devices such as AFCI (Arc Fault Circuit Interrupter) detectiondevices. Ground fault detection devices may be used in any electricalpower delivery system or in any electrical equipment. Accordingly,embodiments of the present invention may be implemented in any type ofground fault detection device.

FIG. 1 illustrates a conventional GFCI device 100 with a supervisorytest circuit that uses the 120 volt AC power supply 101 to directlygenerate the test current stimulus voltage. As illustrated in FIG. 1,the conventional GFCI device 100 includes an application-specificintegrated circuit (ASIC) 102, which includes a differential groundfault detection circuit 104 and circuitry for a self test, such as aself test controller 106 and a timer 108. The conventional GFCI device100 further includes a DC power supply 110 to power the ASIC 102. Adifferential current transformer 112 is built on a toroid 114, with theline conductor 116 and the neutral conductor 118 passing through thetoroid 114 and a secondary winding 120 wound on the toroid 114. A mainmechanical contact switch 122 is provided in the line conductor 116, anda trip solenoid 124 and accompanying electrical switch 126 are used totrip the main mechanical contact switch 122. A high current transientvoltage suppressor component 128 is electrically connected from the lineconductor 116 to the neutral conductor 118, which in this case is alsoan electronics ground. A PTT (Push-To-Test) button 130 can be pushed byan operator to initiate a self test. An alarm 132 alerts the operator ofan automatic self test failure. The supervisory test circuit of the GFCIdevice 100 is comprised of a resistor 134, a third wire 136 passingthrough the toroid 114 of the transformer 112, and an electronic switch138.

Basic function of the GFCI device 100 of FIG. 1 is as follows. Currentleaking from the line conductor 116 on the load side of the mainmechanical contact switch 122 of the device back to ground, or toneutral on the source side of the device through some path other thanthe neutral conductor 118, creates an imbalance of magnetic flux in thetoroid 114 of the differential current transformer 112, causing acurrent to flow in the secondary windings 120. The terminals of thesecondary winding 120 are electrically connected to input pins of thedifferential ground fault detection circuit 104 contained in an ASIC102. A typical detection circuit amplifies the input current signal andcompares the amplitude to a predetermined trip threshold. In devicesthat have automatic self test, the self test controller 106 allows orinhibits the output signal of the detector 104 to pass through to anoutput pin of the ASIC 102. In particular, if a self test is not beingperformed, the self test controller 106 allows the output signal to passthrough to the output pin of the ASIC 102. The output pin of the ASIC102 is electrically connected to a control pin of the electronic switch126, and the output signal (TRIP) is transmitted to the electronicswitch 126. One terminal of the electronic switch 126 is electricallyconnected to the electronics ground. The other terminal of theelectronic switch 126 is electrically connected to one terminal of thetrip solenoid 124. The other terminal of the trip solenoid 124 iselectrically connected to the line conductor 116 of the 120 volts ACpower supply 101 on the load side of main contact switch 122. Tripsolenoid 124 is mechanically located to activate a trip armature thatopens the main contact switch 122 in the line conductor 116 whenenergized.

During normal ground fault detection mode, the control circuit allowsthe output signal of the detector 104 to pass through to an output pinof the ASIC 102. In the case that the detected differential currentexceeds the predetermined trip threshold, the output signal (TRIP) turnson or closes the electronic switch 126 which energizes the trip solenoid124. The trip solenoid 124 activates the trip armature that opens themain contact 122 which interrupts delivery of the 120 volts AC powersupply 101 in the line conductor 116 to the load.

The supervisory test circuit serves to test the health of the groundfault detection device 100. A test may be initiated by the operator bypressing the push-to-test button 130 or may be initiated automaticallyat periodic time intervals triggered by the timer 108. The self testcontroller 106 monitors the push-to-test pin of the ASIC 102 and thetimer 108. One terminal of the PTT button 130 is electrically connectedto the push-to-test pin of the ASIC 102. The other terminal of the PTTbutton 130 is electrically connected to the DC power supply 110. The PTTcircuit can alternatively be configured such that the other terminal isconnected to electronics ground for an active PTT. The control circuitoutputs a signal (GF_TEST) on a pin of the ASIC 102 which iselectrically connected to the control pin of electronic switch 138. Oneterminal of the electronic switch 138 is electrically connected to anelectronics ground. The other terminal of the electronic switch 138 iselectrically connected to one terminal of resistor 134 using a wireconductor 136 that is routed through the toroid 114 of the differentialcurrent transformer 112. The other terminal of the resistor 134 iselectrically connected to the line conductor of the 120 volts AC powersupply 101 on the load side of main contact switch 122. Even with thehigh current transient voltage suppressor component 128 electricallyconnected from the line conductor 116 to the neutral conductor 118, thesupervisory circuit can be exposed to transient voltages with amplitudesof 500 to 600 volts. The supervisory circuit can also be exposed toother low voltage noise on the line conductor that could potentiallyinterfere with the self test.

In the case in which the test is initiated by an operator pressing thepush-to-test button 130, the self test controller 106 transmits a signal(GF_TEST) to turn on or close the electronic switch 138. For example,the particular electronic switch 138 in FIG. 1 can be a siliconcontrolled rectifier (SCR). This switch was selected based on a highvoltage rating of 600 volts to withstand voltages in the line conductorof the 120 volts AC supply 101, and on a package type of SOT-89, whichis one of the smallest size surface mount package offered for such ahigh voltage rated component. The electronic switch 138 behaves like agate controlled diode, except that the switch 138 will stay on once itis turned on for as long as there is a voltage potential drop from theanode to the cathode, even if the gate voltage is removed. Turning on orclosing the electronic switch 138 causes a current signal to flowdirectly from the line conductor of 120 volt AC supply 101 throughresistor 134 and wire 136 which is routed through the toroid 114 of thetransformer 112, and through the electronic switch 138 to electronicsground during the positive half cycle of the 120 volt AC supply. Theamplitude of the test current stimulus signal is set by resistor 134 toa value at least 30% above the trip threshold of the differential groundfault detection circuit 104 in the ASIC 102. For a GFCI device which hasa rated trip current of 6 milliamperes rms, or 8.5 milliamperes peak,the amplitude of the test current is set to just above 8 milliampersrms, or 11.3 milliamperes peak. This is to guarantee detection withmargin when the line conductor of the 120 volts AC supply 101 drops to85%. Intentionally passing the stimulus current through the toroid 114of the transformer 112 creates an imbalance of magnetic flux in thetoroid 114 of the differential current transformer 112, causing acurrent to flow in the secondary windings 120. The current in thesecondary windings 120 is detected by the ground fault detector circuit104 in the ASIC 102.

During a push-to-test, the self test controller 106 can allow the outputsignal of the detector 104 to pass through to an output pin of the ASIC102. The test current stimulus signal generated by the supervisory testcircuit results in a detected differential current that exceeds thepredetermined trip threshold. The detector output signal (TRIP) turns onor closes electronic switch 126 which energizes the trip solenoid 124.The trip solenoid 124 activates a trip armature that opens the maincontact switch 122, which interrupts delivery of the 120 volts AC powersupply 101 in the line conductor 116 to the load. Typically, amechanical switch arm moves from the ON position to a TRIP position,indicating to the operator that the push-to-test has passed. Otherwise,there is no tripping action, indicating to the operator that thepush-to-test has failed.

During an automatic self test, the self test controller 106 inhibits theoutput signal of the detector 104 from passing through to an output pinof the ASIC 102. The test current stimulus signal generated by thesupervisory test circuit results in a detected differential current thatexceeds the predetermined trip threshold. The detector output signal isinhibited by the self test controller 106, preventing the electronicswitch 126 from closing and energizing the trip solenoid 124. Instead,normal operation is resumed. Otherwise, if no differential current isdetected that exceeds the predetermined trip threshold after apredetermined elapsed period of time, the control circuit sends a signalto the alarm circuit 132 to alert the operator that the ground faultdevice is defective and needs to be replaced.

FIG. 2 illustrates a signal timing diagram of the push-to-test self testfor the GFCI device 100 of FIG. 1. In particular, FIG. 2 shows the lineconductor signal 210 of the 120 volts AC power supply 101, thepush-to-test signal 220, the self test timer signal 230, the GF_TESTsignal 240, the test current 250 from the AC power supply 101, the TRIPsignal 260, and the ALARM signal 270 for the case in which the GFCIdevice 100 performs a push-to-test self test. Referring to FIGS. 1 and2, the push-to-test signal 220 is initiated by an operator pushing thePTT button 130 at a random phase of the line conductor 210. The GF_TESTsignal 240 is then generated by the self test controller 106 whichcloses the electronic switch 138 resulting in a test current stimulussignal 250 during the positive half cycle of the line conductor signal210. Therefore, the GF_TEST signal 240 should be present for at leastone complete cycle (e.g., 16.33 milliseconds) of the line conductorsignal 210 to guarantee that a test current stimulus signal 250 will begenerated during the positive half cycle of the line conductor signal totest the ground fault detection circuit. Once the test current signal250 exceeds the detection threshold of the ground fault detector 104,the detector 104 outputs the TRIP signal 260 to trip the circuit breaker(e.g., switch 126).

FIG. 3 illustrates a signal timing diagram of an automatic self test forthe GFCI device 100 of FIG. 1. In particular, FIG. 3 shows the lineconductor signal 310 of the 120 volts AC power supply 101, thepush-to-test signal 320, the self test timer signal 330, the GF_TESTsignal 340, the test current 350 from the AC power supply 101, the TRIPsignal 360, and the ALARM signal 370 for the case in which the GFCIdevice 100 performs an automatic self test. Referring to FIGS. 1 and 3,the self test timer 108 transmits a self test timer signal 330initiating a self test at a random phase of the line conductor 310. TheGF_TEST signal 340 is then generated by the self test controller 106 andcloses the electronic switch 138 resulting in a test current stimulussignal 350 during the positive half cycle of the line conductor signal310. Therefore, the GF_TEST signal 340 should be present for at leastone complete cycle (e.g., 16.33 milliseconds) of the line conductorsignal 310 to guarantee that the test current stimulus signal 350 willbe generated during the positive half cycle of the line conductor signal310 to test the ground fault detection circuit. Once the test currentsignal 350 exceeds the detection threshold of the ground fault detector104, the detector 104 outputs a signal 362 that is inhibited by the selftest controller 106 from reaching the TRIP signal output. However, theself test controller 106 utilizes this signal 362 as a self test passindicator. At the end of the complete cycle self test period, if theself test passed, the self test controller 106 resets the self testtimer 108 and again enables the output of the ground fault detector 104to the output pin of the ASIC. Otherwise, if the self-test fails, theself-test controller outputs an ALARM 370 signal to the alarm circuit132 to alert the operator of a self test failure.

FIG. 4 illustrates a GFCI device 400, according to an embodiment of thepresent invention, that uses a low voltage DC power supply 410 todirectly generate a test current stimulus signal. As illustrated in FIG.4, the GFCI device 400 includes an ASIC 402, which includes adifferential ground fault detection circuit 404 and circuitry for a selftest, such as a self test controller 406 and a timer 408. The GFCIdevice 400 further includes a DC power supply 410 to power the ASIC 402and to directly generate the test current stimulus signal. Adifferential current transformer 412 is built on a toroid 414, with theline conductor 416 and the neutral conductor 418 passing through thetoroid 414 and a secondary winding 420 wound on the toroid 414. A mainmechanical contact switch 422 is provided in the line conductor 416, anda trip solenoid 424 and accompanying electrical switch 426 are used totrip the main mechanical contact switch 422. A high current transientvoltage suppressor component 428 is electrically connected from the lineconductor 416 to the neutral conductor 418, which in the case is alsothe electronics ground. A PTT (Push-To-Test) button 430 can be pushed byan operator to initiate a self test. An alarm 432 alerts the operator ofan automatic self test failure. The supervisory test circuit of the GFCIdevice 400 is comprised of a resistor 434, a third wire 436 passingthrough the toroid 414 of the transformer 412, and an electronic switch438.

According to an embodiment of the present invention, basic function ofthe GFCI device 400 of FIG. 4 is as follows. Current leaking from theline conductor 416 on the load side of the main mechanical contactswitch 422 of the device back to ground, or to neutral on the sourceside of the device through some path other than the neutral conductor418, creates an imbalance of magnetic flux in the toroid 414 of thedifferential current transformer 412, causing a current to flow in thesecondary windings 420. The terminals of the secondary winding 420 areelectrically connected to input pins of the differential ground faultdetection circuit 404 contained in an ASIC 402. According to anadvantageous implementation, the detection circuit 404 may amplify theinput current signal and compare the amplitude to a predetermined tripthreshold. In devices that have automatic self test, the self testcontroller 406 allows or inhibits the output signal of the detector 404to pass through to an output pin of the ASIC 402. In particular, if anautomatic self test is not being performed, the self test controller 406allows the output signal to pass through to the output pin of the ASIC402. The output pin of the ASIC 402 is electrically connected to acontrol pin of the electronic switch 426, and the output signal (TRIP)is transmitted to the electronic switch 426. One terminal of theelectronic switch 426 is electrically connected to the electronicsground. The other terminal of the electronic switch 426 is electricallyconnected to one terminal of the trip solenoid 424. The other terminalof the trip solenoid 424 is electrically connected to the line conductor416 of an AC power supply 401 on the load side of main contact switch422. Trip solenoid 424 is mechanically located to activate a triparmature that opens the main contact switch 422 in the line conductor416 when energized.

During normal ground fault detection mode, the self test controller 406allows the output signal of the detector 404 to pass through to anoutput pin of the ASIC 402. In the case that the detected differentialcurrent exceeds the predetermined trip threshold, the output signal(TRIP) turns on or closes the electronic switch 426 which energizes thetrip solenoid 424. The trip solenoid 424 activates the trip armaturethat opens the main contact 422 which interrupts delivery of the ACpower supply 401 in the line conductor 416 to the load.

The supervisory test circuit serves to test the health of the groundfault detection device 400. A test may be initiated by the operator bypressing the push-to-test button 430 or may be initiated automaticallyat periodic time intervals triggered by the timer 408. The self testcontroller 406 monitors the push-to-test pin of the ASIC 402 and thetimer 408. One terminal of the PTT button 430 is electrically connectedto the push-to-test pin of the ASIC 402. The other terminal of the PTTbutton 430 is electrically connected to the DC power supply 410 (+5VDC). The control circuit outputs a signal (GF_TEST) on a pin of theASIC 402 which is electrically connected to the control pin ofelectronic switch 438. One terminal of the electronic switch 438 iselectrically connected to an electronics ground. The other terminal ofthe electronic switch 438 is electrically connected to one terminal ofresistor 434 using a wire conductor 436 that is routed through thetoroid 414 of the differential current transformer 412. The otherterminal of the resistor 434 is electrically connected to the lowvoltage DC power supply 410. In the embodiment of FIG. 4, the lowvoltage DC power supply 410 supplies +5 VDC, but the present inventionis not limited thereto. The low voltage DC power supply 410 can be abridge rectifier that converts AC power from the AC power supply 401 toDC power 402. The low voltage DC power supply 410 can be implemented asa half wave rectifier, as shown in FIG. 4, but the present invention isnot limited thereto. The low voltage DC power supply 410 filters thehigh voltage transients as well as other low voltage noise on the lineconductor 416 that could potentially interfere with the self test.

In the case in which the test is initiated by an operator pressing thepush-to-test button 430, the self test controller 406 transmits a signal(GF_TEST) to turn on or close the electronic switch 438. According to anadvantageous implementation, the electronic switch 438 in FIG. 4 can beimplemented as SI1902, a dual N-channel MOSFET manufactured by VishaySiliconix. The part was selected by the present inventors based on amuch lower voltage rating requirement of 10 to 20 volts to withstand anytransient voltages that leaked through the low voltage DC power supply410 from the line conductor of the 120 volts AC supply 401, and comes ina very small package type SOT-363, which is 2.1 millimeters by 2millimeters. This is a much smaller package than the SOT-89 for the SCRused in the conventional GFCI device of FIG. 1, which is 4.6 millimetersby 4.25 millimeters. Further, the SI1902 costs less than the SCR, whichcould lead to large cost savings over large quantities in production.The SI1902 electronic switch behaves like a normal switch in that it ison or closed when there is gate voltage, and is off or open when thegate voltage is removed. It is to be understood that the presentinvention is not limited to the use of the SI1902 electronic switch, andthe above description of the SI1902 electronic switch illustrates thatin embodiments of the present invention in which the stimulus signal isgenerated from a low voltage DC power supply, an electronic switch canbe used that has a lower voltage rating, cheaper price, and smallerpackage, as compared with the conventional GFCI device.

Turning on or closing the electronic switch 438 causes a current signalto flow directly from the low voltage DC power supply 410 throughresistor 434 and wire 436 which is routed through the toroid 414 of thetransformer 412, and through the electronic switch 438 to electronicsground. The amplitude of the test current stimulus signal is set byresistor 434 to a value that is slightly above the trip threshold of thedifferential ground fault detection circuit 404 in the ASIC 402. For aGFCI device which has a rated trip current of 6 milliamperes rms, or 8.5milliamperes peak, the amplitude of the test current is set to justabove 8.5 milliamperes DC. It can be noted that any component thatprovides a resistance can be used as the resistor 434 to set theamplitude of the test current stimulus signal. For example, in place atraditional resistor, a field-effect transistor (FET) biased in thelinear region to form a resistance could be used as the resistor 434 toset the amplitude of the test current stimulus signal. Since the testcurrent is directly generated from the low voltage DC power supply 410,the amplitude of the test current stimulus signal remains constant whenthe line conductor 416 of the 120 volts AC power supply 401 drops to85%. Accordingly, there is no need to set the amplitude of the testcurrent stimulus signal at least 30% above the trip threshold of thedifferential ground fault detection circuit 404 in the ASIC 402, as inthe conventional GFCI device, to guarantee detection with margin whenthe line conductor of the 120 volts AC supply drops to 85%. Thiseliminates the possibility of a test escape in that if the tripthreshold of the differential ground fault detection circuit 404 in theASIC 402 degrades by 20%, the self test will fail, as it should. In theconventional GFCI device of FIG. 1, the self test could still pass eventhough the ground fault detection device degraded to a point that isnoncompliant with UL (Underwriters Laboratory) requirements. The amountof momentary power dissipated in the supervisory circuit is reduced from960 milliwatts in the conventional GFCI device of FIG. 1 to 43milliwatts in the GFCI device 400 of FIG. 4. This allows one to uselower power rated components for the resistor 434 and the electronicswitch 438 and still improve component reliability. This is importantfor devices that include automatic self test since the supervisory testcircuit will be exercised periodically over 50,000 times over a lifetimeof 20 years. Intentionally passing the stimulus current through thetoroid 414 of the transformer 412 creates an imbalance of magnetic fluxin the toroid 414 of the differential current transformer 412, causing acurrent to flow in the secondary windings 420. The current in thesecondary windings 420 is detected by the ground fault detector circuit404 in the ASIC 402.

During a push-to-test, the self test controller 406 can allow the outputsignal of the detector 404 to pass through to an output pin of the ASIC402. The test current stimulus signal generated by the supervisory testcircuit results in a detected differential current that exceeds thepredetermined trip threshold. The detector output signal (TRIP) turns onor closes electronic switch 426 which energizes the trip solenoid 424.The trip solenoid 424 activates a trip armature that opens the maincontact switch 422, which interrupts delivery of the 420 volts AC powersupply 401 in the line conductor 416 to the load. Typically, amechanical switch arm moves from the ON position to a TRIP position,indicating to the operator that the push-to-test has passed. Otherwise,there is no tripping action, indicating to the operator that thepush-to-test has failed.

During an automatic self test, the self test controller 406 inhibits theoutput signal of the detector 404 from passing through to an output pinof the ASIC 402. The test current stimulus signal generated by thesupervisory test circuit results in a detected differential current thatexceeds the predetermined trip threshold. The detector output signal isinhibited by the self test controller 406, preventing the electronicswitch 426 from closing and energizing the trip solenoid 424. Instead,normal operation is resumed. Otherwise, if no differential current isdetected that exceeds the predetermined trip threshold after apredetermined elapsed period of time, the control circuit sends a signalto the alarm circuit 432 to alert the operator that the ground faultdevice is defective and needs to be replaced.

FIG. 5 illustrates a signal timing diagram of the push-to-test self testfor the GFCI device 400 of the embodiment of FIG. 4. In particular, FIG.5 shows the line conductor signal 510 of the 120 volts AC power supply501, the push-to-test signal 520, the self test timer signal 530, theGF_TEST signal 540, the test current 550 from the low voltage DC powersupply 410, the TRIP signal 560, and the ALARM signal 570 for the casein which the GFCI device 400 performs a push-to-test self test.Referring to FIGS. 4 and 5, the push-to-test signal 520 is initiated byan operator pushing the PTT button 430 at a random phase of the lineconductor signal 510. The GF_TEST signal 540 is then generated by theself test controller 406 which closes the electronic switch 438 of thesupervisory test circuit resulting in a test current stimulus signal 550(e.g., 8.5 milliamperes) on wire 436 from the low voltage DC powersource 410. The GF_TEST signal 540 should be present for at least onecomplete cycle (e.g., 16.33 milliseconds) of the line conductor signal510 to guarantee that a test current stimulus signal 550 will begenerated that exceeds the trip threshold to test the ground faultdetection circuit in case there is any minimal leakage of ground faultcurrent (less than 6 milliamperes rms, 8.5 milliamperes peak) during thenegative half cycle. Once the test current signal 550 exceeds thedetection threshold of the ground fault detector 404, the detector 404outputs the TRIP signal 560 to trip the circuit breaker (e.g., switch426).

FIG. 6 illustrates a signal timing diagram of an automatic self test forthe GFCI device 400 according to the embodiment of FIG. 4. Inparticular, FIG. 6 shows the line conductor signal 610 of the 120 voltsAC power supply 401, the push-to-test signal 620, the self test timersignal 630, the GF_TEST signal 640, the test current 650 from the lowvoltage DC power supply 410, the TRIP signal 660, and the ALARM signal670 for the case in which the GFCI device 400 performs an automatic selftest. Referring to FIGS. 4 and 6, the self test timer 408 transmits aself test timer signal 630 initiating a self test at a random phase ofthe line conductor signal 610. The GF_TEST signal 640 is then generatedby the self test controller 406 and closes the electronic switch 438resulting in a test current stimulus signal 650 (e.g., 8.5 milliamperes)on wire 436 from the low voltage DC power source 410. The GF_TEST signal640 should be present for at least one complete cycle (e.g., 16.33milliseconds) of the line conductor signal 610 to guarantee that a testcurrent stimulus signal 650 will be generated that exceeds the tripthreshold in case there is any minimal leakage of ground fault current(less than 6 milliamperes rms, 8.5 milliamperes peak) during thenegative half cycle. Once the test current signal 650 exceeds thedetection threshold of the ground fault detector 404, the detector 404outputs a signal 662 that is inhibited by the self test controller 406from reaching the TRIP signal output. However, the self test controller406 utilizes this signal 662 as a self test pass indicator. At the endof the complete cycle self test period, if the self test passed, theself test controller 406 resets the self test timer 408 and againenables the output of the ground fault detector 404 to the output pin ofthe ASIC. Otherwise, if the self-test fails, the self-test controller406 outputs an ALARM signal 670 to the alarm circuit 432 to alert theoperator of a self test failure.

FIG. 7 illustrates a GFCI device 700, according to another embodiment ofthe present invention. In the embodiment of FIG. 7, the requiredduration of the test current stimulus signal can be reduced from acomplete cycle (e.g., 16.33 milliseconds) of the line conductor 716 ofthe AC power supply 701 to a few milliseconds by synchronizing the testcurrent stimulus signal to the line conductor of 716 of the AC powersupply 701. The components 701-738 of the GFCI device 700 of FIG. 7operate similarly to the respective components 401-438 of the GFCIdevice 400 of FIG. 4 described above, other than the followingdifferences described hereinafter.

The GFCI device 700 of FIG. 7 is similar to the GFCI device 400 of FIG.4, but includes a resistor 742 that electrically couples the lineconductor voltage of the AC power supply 701 to the ASIC 702. The ASIC702 includes a line synchronizer circuit 740 to synchronize the testcurrent stimulus signal to the line conductor of the AC power supply701. One terminal of resistor 742 is electrically connected to the LINEconductor of the 120 volt AC supply 701. The other terminal iselectrically connected to a pin of the ASIC 702 that is input into theline synchronizer circuit 740 The line synchronizer circuit 740 outputsa signal to the self test controller 706 when the amplitude exceeds apredetermined threshold during the positive half cycle of the lineconductor of the 120 volt AC power supply 701. It is to be understood,that the line synchronizer circuit 740 can output the signal to the selftest controller 706 when the amplitude exceeds a predetermined thresholdduring the negative half cycle of the line conductor of the 120 volt ACpower supply 701, in a cased in which the wire conductor 736 is routedin the opposite direction through the toroid 714 of the transformer 712.The self test controller 706 utilizes this signal to determine when togenerate the GF_TEST signal during a self test. In the embodiment ofFIG. 7, the push-to-test self test and the automatic self test operatein a similar manner as described above with respect to FIG. 4. However,the timing diagram is different with the self test controller 706utilizing the output signal from the line synchronizer circuit 740, asin the embodiment of FIG. 7.

FIG. 8 illustrates a signal timing diagram of the push-to-test self testfor the GFCI device 700 of the embodiment of FIG. 7. In particular, FIG.8 shows the line conductor signal 810 of the 120 volts AC power supply701, the push-to-test signal 820, the self test timer signal 830, theGF_TEST signal 840, the test current 850 from the low voltage DC powersupply 710, the TRIP signal 860, and the ALARM signal 870 for the casein which the GFCI device 700 performs a push-to-test self test.Referring to FIGS. 7 and 8, the push-to-test signal 820 is initiated byan operator pushing the PTT button 730 at a random phase of the lineconductor signal 810. The GF_TEST signal 840 is then generated by theself test controller 706 which closes the electronic switch 738 of thesupervisory test circuit resulting in a test current stimulus signal 850(e.g., 8.5 milliamperes) on wire 736 from the low voltage DC powersource 710. The self test controller 706 gates the output signal fromthe line synchronizer circuit 740 so that the GF_TEST signal is presentfor only a few milliseconds during the positive half cycle of the lineconductor signal 810 of the AC power supply 701. This is the optimaltime to generate the test current stimulus signal 850. It is to beunderstood that in an alternative embodiment, the GF_Test signal couldbe present during the negative half cycle of the line conductor signal,in a case in which the wire conductor 736 is routed in the oppositedirection through the toroid 714. Once the test current signal 850exceeds the detection threshold of the ground fault detector 704, thedetector 704 outputs the TRIP signal 860 to trip the circuit breaker(e.g., switch 726).

FIG. 9 illustrates a signal timing diagram of an automatic self test forthe GFCI device 700 according to the embodiment of FIG. 7. Inparticular, FIG. 9 shows the line conductor signal 910 of the 120 voltsAC power supply 701, the push-to-test signal 920, the self test timersignal 930, the GF_TEST signal 940, the test current 950 from the lowvoltage DC power supply 710, the TRIP signal 960, and the ALARM signal970 for the case in which the GFCI device 700 performs an automatic selftest. Referring to FIGS. 7 and 9, the self test timer 708 transmits aself test timer signal 930 initiating a self test at a random phase ofthe line conductor signal 910. The GF_TEST signal 940 is then generatedby the self test controller 706 and closes the electronic switch 738resulting in a test current stimulus signal 950 (e.g., 8.5 milliamperes)on wire 736 from the low voltage DC power source 710. The self testcontroller 706 gates the output signal from the line synchronizercircuit 740 so that the GF_TEST signal is present for only a fewmilliseconds during the positive half cycle of the line conductor signal910 of the AC supply 701. This is the optimal time to generate the testcurrent stimulus signal 950. It is to be understood that in analternative embodiment, the GF_Test signal could be present during thenegative half cycle of the line conductor signal, in a case in which thewire conductor 736 is routed in the opposite direction through thetoroid 714. Once the test current signal 950 exceeds the detectionthreshold of the ground fault detector 704, the detector 704 outputs asignal 962 that is inhibited by the self test controller 706 fromreaching the TRIP signal output. However, the self test controller 706utilizes this signal 962 as a self test pass indicator. At the end ofthe self test period, if the self test passed, the self test controller706 resets the self test timer 708 and again enables the output of theground fault detector 704 to the output pin of the ASIC 702. Otherwise,if the self-test fails, the self-test controller 706 outputs an ALARMsignal 970 to the alarm circuit 732 to alert the operator of a self testfailure.

FIG. 10 illustrates a GFCI device 1000, according to another embodimentof the present invention. It is to be understood that the components1001-1042 of the GFCI device 1000 of FIG. 10 operate similarly to therespective components 701-742 of the GFCI device 700 of FIG. 7 describedabove. The elements the supervisory circuit including the electronicswitch 1038, the wire conductor 1036 that is routed through the toroid1014 of the differential current transformer 1012, and the resistor 1034that sets the amplitude of the test current stimulus signal areconnected electrically in series. These elements, which are electricallyconnected in series, can be electrically connected in any order. Forexample, the embodiment of FIG. 10 swaps the order of the electronicswitch 1038 and the wire conductor 1036 that is routed through thetoroid 1014 of the differential current transformer 1012, as comparedwith the electronic switch (438, 738) and wire conductor (436, 736) inprevious embodiments.

FIG. 11 illustrates a GFCI device 1100, according to another embodimentof the present invention. It is to be understood that the components1101-1142 of the GFCI device 1100 of FIG. 11 operate similarly to therespective components 701-742 of the GFCI device 700 of FIG. 7 describedabove. The embodiment of FIG. 11 swaps the order of the electronicswitch 1138 and resistor 1134, as compared with the electronic switch1038 and resistor 1034 in the embodiment of FIG. 10.

FIG. 12 illustrates a GFCI device 1200, according to another embodimentof the present invention. It is to be understood that the components1201-1242 of the GFCI device 1200 of FIG. 12 operate similarly to therespective components 701-742 of the GFCI device 700 of FIG. 7 describedabove. In the embodiment of FIG. 12, the electronic switch 1238 isintegrated into the ASIC 1202. As described above, various embodimentsof the present invention enable a low voltage rated electronic switch tobe used for the electronic switch 1238. This allows the electronicswitch 1238 to be integrated into a low power, low voltage CMOS ASIC1202, which can reduce cost and board space. In addition, theintegration of the electronic switch 1238 into the ASIC 1202 can be donewithout adding any additional pins to the ASIC 102 by reusing the pinfor the GF_TEST signal.

FIG. 13 illustrates a GFCI device 1300, according to another embodimentof the present invention. It is to be understood that the components1301-1342 of the GFCI device 1300 of FIG. 13 operate similarly to therespective components 701-742 of the GFCI device 700 of FIG. 7 describedabove. In the embodiment of FIG. 13, the electronic switch 1338 and theresistor 1234 are integrated into the ASIC 1302. The integration of theresistor 1234 into the ASIC 1202 along with the electronic switch 1338can result in an additional reduction of cost and board space. As shownin FIG. 13, in this embodiment, the DC test stimulus signal is generatedinside the ASIC 1302 from the DC power supplied to the ASIC 1302.Furthermore, although FIG. 13 shows the DC power supply 1310 beingseparate from the ASCI 1302, the present invention is not limitedthereto. In another possible embodiment, the DC power supply circuitrycan be included inside the ASIC instead of separate from the ASIC.

FIG. 14 illustrates a method of performing a self test by a GFCI deviceaccording to an embodiment of the present invention. The method of FIG.14 can be performed by the GFCI devices illustrated in FIGS. 4, 7, 10,11, 12, and 13. The self-test can be an automatic self test or a“push-to-test” self test. As illustrated in FIG. 14, at step 1402 theself test is initiated. In the case of the push-to-test self test, theself test is initiated by a user pressing the push-to-test button, whichcauses the push-to-test signal to be sent to the self test controller.In the case of an automatic self test, the self test timer sends asignal to the self test controller to initiate the self test. At step1404, a test stimulus signal is generated in the wire conductor routedthrough the toroid of the differential current transformer from the lowvoltage DC power source. In particular, the self-test controller cancontrol the electronic switch electrically connected to the wireconductor and the low voltage power supply to close causing the teststimulus signal to flow from the low voltage power supply through thewire conductor, and a resistor can control the amplitude of the teststimulus signal from the low voltage power supply. At step 1406, it isdetermined whether a differential current exceeding the trip thresholdis detected by the differential ground fault detector. At step 1408, ifa differential current exceeding the trip threshold is detected at step1406, the self test passes. In the case of the push-to-test self test,when the differential current exceeding the trip threshold is detected,the TRIP signal is sent to an electronic switch which energizes a tripsolenoid that activates the trip armature to open the main contact whichinterrupts delivery of the AC power in the line conductor. In the caseof an automatic self test, when the differential current exceeding thetrip threshold is detected, the TRIP signal is suppressed by the selftest controller, and the self test timer is reset. At step 1410, if adifferential threshold exceeding the trip threshold is not detected atstep 1406, the self test fails. In the push-to-test self test, when theself test fails, no TRIP signal is generated and the main contact is notopened, which alerts the user that the test has failed. In the automaticself-test, an alarm signal is sent to the alarm circuit to alert a userthat the test has failed.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDetailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. It is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present invention and that variousmodifications may be implemented by those skilled in the art withoutdeparting from the scope and spirit of the invention. Those skilled inthe art could implement various other feature combinations withoutdeparting from the scope and spirit of the invention.

1. A ground fault detection device comprising: line and neutralconductors configured to connect an AC power source and a load; adifferential current transformer comprising a toroid, through which theline and neutral conductors pass, and a secondary winding wound on thetoroid; a differential ground fault detector electrically connected tothe secondary winding of the differential current transformer, thedifferential ground fault detector configured to compare currentgenerated in the secondary winding from an imbalance of magnetic flux inthe toroid to a predetermined threshold; a wire conductor routed throughthe toroid of the differential current transformer; and a controllerconfigured to control a low voltage DC test stimulus signal to begenerated in the wire conductor.
 2. The ground fault detection device ofclaim 1, further comprising: a low voltage DC power supply electricallyconnected to the wire conductor, wherein the controller is configured tocontrol the low voltage DC power supply to generate the test stimulussignal in the wire conductor.
 3. The ground fault detection device ofclaim 1, further comprising: an application specific integrated circuit(ASIC), wherein the differential ground circuit fault detector and thecontroller are included on the ASIC.
 4. The ground fault detectiondevice of claim 3, further comprising: a low voltage DC power supplyelectrically connected to the wire conductor to generate the teststimulus signal in the wire conductor, and electrically connected to theASIC to supply power to the ASIC.
 5. The ground fault detection deviceof claim 1, further comprising: an electronic switch electricallyconnected to the wire conductor and the controller, wherein thecontroller is configured to control the low voltage DC test stimulussignal to be generated in the wire conductor by transmitting a signal tothe electronic switch to close the electronic switch.
 6. The groundfault detection device of claim 5, wherein the controller is configuredto transmit the signal to the electronic switch to close the electronicswitch for at least one complete cycle of the AC power on the lineconductor.
 7. The ground fault detection device of claim 5, furthercomprising: a line synchronizer electrically connected to the lineconductor and the controller and configured to synchronize thecontroller with the line conductor, wherein the controller is configuredto transmit the signal to the electronic switch to close the electronicswitch during a positive half cycle of the AC power on the lineconductor.
 8. The ground fault detection device of claim 5, furthercomprising: a line synchronizer electrically connected to the lineconductor and the controller and configured to synchronize thecontroller with the line conductor, wherein the controller is configuredto transmit the signal to the electronic switch to close the electronicswitch during a negative half cycle of the AC power on the lineconductor.
 9. The ground fault detection device of claim 5, furthercomprising: an application specific integrated circuit (ASIC), whereinthe differential ground circuit fault detector, the controller, and theelectronic switch are included on the ASIC.
 10. The ground faultdetection device of claim 5, further comprising: a resistor electricallyconnected to the wire conductor, wherein the resistor is configured toset an amplitude of the low voltage DC test stimulus signal.
 11. Theground fault detection device of claim 10, further comprising: anapplication specific integrated circuit (ASIC), wherein the differentialground circuit fault detector, the controller, the electronic switch,and the resistor are included on the ASIC.
 12. The ground faultdetection device of claim 11, the ASIC further includes a low voltage DCpower source electrically connected to at least one of the electronicswitch and the resistor and configured to generate the low voltage DCtest stimulus signal in the wire conductor.
 13. The ground faultdetection device of claim 1, further comprising: a push-to-test button,electrically connected to the controller and configured to transmit asignal to the controller in response to a user pressing the push-to-testbutton, wherein the controller is configured to control the low voltageDC test stimulus signal to be generated in the wire conductor inresponse to receiving the signal from the push-to-test button.
 14. Theground fault circuit interrupt device of claim 1, further comprising: aself test timer electrically connected to the controller and configuredto automatically transmit a signal to the controller, wherein thecontroller is configured to control the low voltage DC test stimulussignal to be generated in the wire conductor in response to receivingthe signal from the self-test timer.
 15. The ground fault detectordevice of claim 1, wherein the differential ground fault detector isconfigured to transmit a trip signal to the controller in response todetecting that the current on the secondary windings is greater than thepredetermined threshold.
 16. The ground fault detector device of claim15, further comprising: an alarm circuit configured to generate an alertthat the ground fault circuit interrupt device is defective, wherein thecontroller is configured to transmit an alarm signal to the alarmcircuit when the test current stimulus signal is generated in the wireconductor and the trip signal is not received from the differentialground fault detector.
 17. A method of performing a self test by aground fault circuit interrupt device, comprising: generating a lowvoltage DC test stimulus signal in a wire conductor routed through atoroid of a differential current transformer; and determining whether adifferential current greater than a predetermined threshold is detectedin the toroid.
 18. The method of claim 17, further comprising: receivingpush-to-test signal from a push-to-test button, wherein the step ofgenerating a low voltage DC test stimulus signal is performed inresponse to receiving the push-to-test signal.
 19. The method of claim18, further comprising: if the differential current greater than thepredetermined threshold is detected in the toroid, tripping a maincontact switch of the ground fault circuit interrupt device.
 20. Themethod of claim 17, further comprising: receiving a self test timersignal from a self test timer, wherein the step of generating a lowvoltage DC test stimulus signal is performed in response to receivingthe self test timer signal.
 21. The method of claim 20, furthercomprising: if the differential current greater than the predeterminedthreshold is detected in the toroid, suppressing a trip signal thattrips a main contact switch of the ground fault circuit interrupt deviceand resetting the self test timer; and if the differential currentgreater than the predetermined threshold is not detected in the toroid,generating an alarm to alert a user that the ground fault circuitinterrupt device is defective.
 22. The method of claim 17, wherein thestep of generating a low voltage DC test stimulus signal in a wireconductor routed through a toroid of a differential current transformercomprises: generating the low voltage DC test stimulus signal in thewire conductor during a portion of a positive half cycle of an AC powersignal in a line conductor of the ground fault circuit interrupt device.23. The method of claim 17, wherein the step of generating a low voltageDC test stimulus signal in a wire conductor routed through a toroid of adifferential current transformer comprises: generating the low voltageDC test stimulus signal in the wire conductor during a portion of anegative half cycle of an AC power signal in a line conductor of theground fault circuit interrupt device.
 24. A ground fault detectiondevice comprising: means for generating a low voltage DC test stimulussignal in a wire conductor routed through a toroid of a differentialcurrent transformer; and means for determining whether a differentialcurrent greater than a predetermined threshold is detected in thetoroid.
 25. The ground fault detection device of claim 24, wherein themeans for generating a low voltage DC test stimulus signal comprises:means for generating the low voltage DC test stimulus signal in responseto receiving a push-to-test signal from a push-to-test button.
 26. Theground fault detection device of claim 25, further comprising: means fortripping a main contact switch when the differential current greaterthan the predetermined threshold is detected in the toroid.
 27. Theground fault detection device of claim 24, wherein the means forgenerating a low voltage DC test stimulus signal comprises: means forgenerating the low voltage DC test stimulus signal in response toreceiving a self test timer signal from a self test timer.
 28. Theground fault detection device of claim 27, further comprising: means forsuppressing a trip signal that trips a main contact switch when thedifferential current greater than the predetermined threshold isdetected in the toroid; and means for generating an alarm to alert auser that the ground fault circuit interrupt device is defective whenthe differential current greater than the predetermined threshold is notdetected in the toroid.
 29. The ground fault detection device of claim24, wherein the means for generating a low voltage DC test stimulussignal in a wire conductor routed through a toroid of a differentialcurrent transformer comprises: means for generating the test stimulussignal in the wire conductor from the low voltage DC power supply duringa portion of a positive half cycle of an AC power signal in a lineconductor of the ground fault circuit interrupt device.
 30. The groundfault detection device of claim 24, wherein the means for generating alow voltage DC test stimulus signal in a wire conductor routed through atoroid of a differential current transformer comprises: means forgenerating the test stimulus signal in the wire conductor from the lowvoltage DC power supply during a portion of a negative half cycle of anAC power signal in a line conductor of the ground fault circuitinterrupt device.